It is necessary to produce periodically an essentially disinfected condition in soft contact lenses so that bacterial organisms or their by-products will not cause harm to the wearer's eyes. Since the soft lens material is permeable to liquids, soaking the lens in strong germicidal solutions will result in the lens becoming impregnated with the solution, and this can lead to irritation to the user's eye when the lens is worn. In general, it has been found difficult, if not impossible, to disinfect soft lens by treatment with chemical or biochemical solutions which will not cause eye irritation to at least some percentage of the wearers.
As an alternate means for producing the desired disinfected condition in the soft lens, heat may be used. The lens must be kept immersed in physiologically normal saline solution, or its equivalent, when it is not being worn, to prevent the lens material from drying out. Heat is therefore generally applied by first placing the lens in a suitable container or lens holder; adding a suitable amount of saline solution, of the proper concentration, so that the lens is totally immersed; closing or covering the lens container; placing the lens container in a suitable heating unit; and energizing the heating unit.
The heating unit must raise the temperature of the saline solution and immersed lens to the required temperature, hold the lens at or above this termperature for the required time, and then allow it to cool to ambient temperature. Typical values of the time and temperature deemed suitable for producing the disinfected condition require the lens to be maintained at or above 80.degree. C., for a period of 10 minutes or more. Since aging of the lens material is accelerated by excessive temperatures and/or by extended time at elevated temperatures, it is desirable that the heating unit be controlled so that excessive temperatures, or excessive time at elevated temperatures, will not shorten the life of the lens.
It is also highly desirable that the lens user be able to check periodically on the proper operation of the heating unit to insure that the unit has reached the desired temperature. An indicator light on a typical prior art heating unit only tells the user that the unit is connected to a functioning electrical source, and that the unit has been turned on. The light may well function normally even if the heating element in the prior art unit is defective and the lens has not reached the proper temperature.
The heating unit must be designed with careful attention to electrical safety/shock hazard considerations since the user typically uses the unit in a bathroom adjacent to grounded water piping and wash basins. Since the typical household electrical outlets are only two contact, without grounding pin provisions, it is impractical to use a 3-wire power cord with a ground connection to any exposed metallic portions of the device. The user of the prior art unit is therefore potentially subject to a significant shock hazard if he touches a grounded object such as a water faucet while in contact with any metallic portion of the heating unit. Any current leakage path between the electrical circuitry and the exposed metallic portion of the prior art unit can lead to potentially fatal shock hazards under these conditions. It would therefore be very advantageous to have the heating unit constructed so that there is no exposed or exposable portion of the unit constructed of metal or other electrically conductive materials. The Co-pending application Ser. No. 834,104 discloses such a heating unit.
An objective of the present invention is to provide in a heating unit of the type disclosed in the copending application, a simple and inexpensive means for the user to observe when the proper temperature has been reached during the operation of the unit, and a postive end-of-heating-cycle indication, readily visible to the user.